The present disclosure relates to electrostatic latent image developing toners and methods for producing such a toner.
From a viewpoint of energy saving and miniaturization of image forming apparatuses, a toner should preferably have excellent low-temperature fixability. The toner having the excellent low-temperature fixability can be satisfactorily fixed on a recording medium even when the temperature of a fixing roller is low.
In order to produce such a toner excellent in the low-temperature fixability, a toner producing method has been proposed that uses a binder resin having a low melting point or a binder resin having a low glass transition point, and a releasing agent having a low melting point. However, it is difficult to produce a toner excellent in high-temperature preservability by this method. The high-temperature preservability refers to a property of a toner that toner particles contained in the toner are not aggregated even when the toner is stored in a high temperature environment. The toner particles of a toner poor in the high-temperature preservability tends to be aggregated in a high temperature environment. When the toner particles aggregate, the chargeable amount of the toner particles may be liable to decrease.
For purpose of improving the low-temperature fixability, high-temperature preservability, and blocking resistance of a toner, a toner containing a toner particle having a core-shell structure has been proposed.
In an example toner containing a toner particle having a core-shell structure, a toner core contains a binder resin having a low melting point. The toner core is covered with a shell layer made from a resin. Further, the resin constituting the shell layer has a glass transition point (Tg) higher than the binder resin contained in the toner core.
Another example toner containing a toner particle having a core-shell structure has a toner core having a surface covered with a thin film (shell layer) containing a thermosetting resin. The toner core has a softening point of 40° C. or more and 150° C. or less.
The toner particle contained in a toner may be treated with an external additive in some cases. For example, in order to provide fluidity to the toner, provide favorable chargeability to the toner particle, or facilitate cleaning of the toner particles adhering to the surface of a photosensitive drum, inorganic powder of silica, titanium oxide, or the like may be caused to adhere to the surface of a toner mother particle.
When an image is formed for a long term using the toner containing toner particles to which the inorganic powder is externally added, the toner continues to be stirred in a developing device to receive stress. This may bury the external additive in the surface of the toner particles. When the external additive is buried in the surface of the toner particles, the fluidity of the toner may decrease, and therefore, various types of image defects may tend to be caused.
In order to enhance the fluidity and durability of a toner, a toner has been proposed in which an external additive having a specific shape adheres to toner mother particles containing a colorant. The proposed toner contains, as an external additive in the toner particles, particulates of rutile titanium oxide in a non-spherical shape having a number average dispersion particle size of a major axis diameter of 0.03-0.5 μm and a minor axis diameter of 0.01-0.2 μm. The surface of each rutile titanium oxide particulate is treated with a hydrophobization agent such as a coupling agent.